1. Field
This invention pertains to maintenance and operation methods to clean and operate reverse osmosis filtration systems by sulfurous acid pre-treatment to condition the brines for heavy metals removal. In particular it pertains to a method utilizing sulfurous acid to reduce scaling and microbial buildup on reverse osmosis membranes to improve their performance to provide product water filtrate, and remove the heavy metals from the brine retentates to enable them to be re-used without heavy metals exposure to those coming into contact with the retentates.
2. State of the Art
Reverse osmosis is a separation process using pressure to force a solvent through a membrane retaining the solute on one side and allowing the pure solvent to pass to the other side. More formally, it is the process of forcing a solvent from a region of high solute concentration through a membrane to a region of low solute concentration by applying a pressure in excess of the Osmotic pressure. This is the reverse of the normal osmosis process, which is the natural movement of solvent from an area of low solute concentration, through a membrane, to an area of high solute concentration when no external pressure is applied. The membrane here is semi permeable, meaning it allows the passage of solvent but not of solute.
Reverse osmosis thus operates using a membrane separation process for removing solvent from a solution. When a semi permeable membrane separates a dilute solution from a concentrated solution, solvent crosses from the dilute to the concentrated side of the membrane in an attempt to equalize concentrations. The flow of solvent can be prevented by applying an opposing hydrostatic pressure to the concentrated solution. The magnitude of the pressure required to impede completely the flow of solvent is defined as the “osmotic pressure”. If the applied hydrostatic pressure exceeds the osmotic pressure, flow of solvent will be reversed, whereby solvent will flow from the concentrated to the dilute solution. This phenomenon is referred to as Reverse Osmosis.
In order to use reverse osmosis as a water purification process, the feed water is pressurized on one side of a semi permeable membrane. The pressure must be high enough to exceed the osmotic pressure to cause reverse osmotic flow of water. If the membrane is highly permeable to water, but essentially impermeable to dissolved solutes, pure water crosses the membrane and is known as product water. As product water crosses the membrane, the concentration of dissolved impurities increases in the remaining feed water (a condition known as concentration polarization) and, as a consequence, the osmotic pressure increases proportional to the concentrations of the constituents.
The osmotic pressure (π) of a solution can be determined experimentally by measuring the concentrations of dissolved salts in the solution. The osmotic pressure is obtained from the following equation.π=RTΣXi
Where
π is the osmotic pressure in kilopascals (kPa)
T is the temperature in Kelvin
R is the Universal gas constant equal to 8.314 kPa m3/kg mol K
ΣXi is the concentration of all constituents in a solution in kg mol/m3 
A point is reached where the applied pressure is no longer able to overcome the osmotic pressure and no further flow of product water occurs. Moreover, if the applied pressure is increased in an attempt to gain more product water, a point is reached when the membrane becomes fouled by precipitated salts and other undissolved material from the water. Therefore, a limit exists as to the fraction of feed water recoverable as pure water and reverse osmosis units are operated in a configuration where only a portion of the feed water passes through the membrane with the remainder being directed to drain (cross-flow configuration).
The water flowing to drain contains concentrated solutes and other insoluble materials, such as bacteria, endotoxin and particles, and is referred to as the reject stream. The product water to feed water ratio can range from 10% to 50% for purification of water depending on the characteristics of the incoming water plus other conditions.
The membranes used for reverse osmosis have a dense barrier layer in the polymer matrix where most separation occurs. In most cases the membrane is designed to allow only water to pass through this dense layer while preventing the passage of solutes (such as salt ions). This process requires a high pressure be exerted on the high concentration side of the membrane, usually 2-17 bar (30-250 psi) for fresh and brackish water, and 40-70 bar (600-1000 psi) for seawater, which has around 24 bar (350 psi) natural osmotic pressure which must be overcome.
A reverse osmosis membrane must be freely permeable to water, highly impermeable to solutes, and able to withstand high operating pressures. It should ideally be tolerant of wide ranges of pH and temperature and should be resistant to attack by chemicals including free chlorine and by bacteria. Ideally, it should also be resistant to scaling and fouling by contaminants in the feed water. Three major types of reverse osmosis membranes: cellulosic, fully aromatic polyamide, and thin film composite. A comparison of characteristics of these three membrane types is shown below.
Comparison of Reverse Osmosis MembranesFeaturesComposite*CellulosicAromatic PolyamideThin FilmRejectionLowMediumHighof OrganicRejection of LowMediumHighHighMolecular WeightOrganic CompoundsWater FluxMediumLowHighpH Tolerance4-84-112-11Maximum35° C.35° C.45° C.TemperatureStabilityOxidantHighLowLowTolerancee.g. free ChlorineCompactionHighHighLowTendencyBiodegradabilityHighLowLowCostLowMediumHigh*Thin film composite type having polyamide surface layer
Cellulosic Membranes were first used in the late 1950s with cellulose acetate membranes. These membranes are asymmetric, composed of a thin dense surface layer (0.2 to 0.5 ˜m) and a thick porous substructure. Solute rejection is accomplished by the thin dense layer and the porous substructure provides structural strength. Cellulose acetate membranes can be cast in sheets or as hollow fibers. Cellulose acetate membranes are inexpensive and easy to manufacture but suffer from several limitations. Their asymmetric structure makes them susceptible to compaction under high operating pressures, especially at elevated temperatures. Compaction occurs when the thin dense layer of the membrane thickens by merging with the thicker porous substructure, leading to a reduction in product flux. Cellulose acetate membranes are susceptible to hydrolysis and can only be used over a limited pH range (low pH 3 to 5 and high pH 6 to 8, depending on the manufacturers). They also undergo degradation at temperatures above 35° C. They are also vulnerable to attack by bacteria. Cellulose acetate membranes have high water permeability but reject low molecular weight contaminants poorly. Cellulose triacetate membranes have been developed with improved salt rejection characteristics and reduced susceptibility to pH, high temperature and microbial attack. However, cellulose triacetate membranes have lower water permeability than cellulose acetate membranes. Blends of cellulose triacetate and cellulose acetate have been developed to take advantage of the desirable characteristics of both membranes.
Aromatic polyamide membranes were first developed by DuPont in a hollow fiber configuration. Similar to the cellulosic membranes, these membranes have an asymmetric structure with a thin (0.1 to 1.0 μm) dense skin and a porous substructure. Polyamide membranes have better resistance to hydrolysis and biological attack than do cellulosic membranes. They can be operated over a pH range of 4 to 11, but extended use at the extremes of this range can cause irreversible membrane degradation. They can withstand higher temperatures than cellulosic membranes. However, similar to cellulosics, they are subject to compaction at high pressures and temperatures. They have better salt rejection characteristics than cellulosic membranes plus better rejection of water soluble organic compounds. A major drawback of polyamide membranes is they are subject to degradation by oxidants, such as free chlorine.
Thin film composites (TFC) are membranes made by forming a thin, dense, solute rejecting surface film on top of a porous substructure. The materials of construction and the manufacturing processes for these two layers can be different and optimized for the best combination of high water flux and low solute permeability. The water flux and solute rejection characteristics are predominantly determined by the thin surface layer, whose thickness ranges from 0.01 to 0.1 micrometers. Several types of thin film composite membranes have been developed, including aromatic polyamide, alkyl-aryl poly urea/polyamide and polyfurane cyanurate. The supporting porous sub layer is usually made of polysulfone. Polyamide thin film composites similar to polyamide asymmetric membranes are highly susceptible to degradation by oxidants, such as free chlorine. Consumers must be consistent in their maintenance of the TFC systems, particularly the carbon pre filtration element essential to be present to remove free chlorine (and other oxidative organic compounds) and prevent damage and premature destruction of the TFC membrane. Although the stability of these membranes to free chlorine has been improved by modifications of the polymer formulation and the processing technique, exposure to oxidants must be minimized.
Reverse osmosis membranes reject dissolved inorganic solutes, larger organic solutes (molecular weight greater than 200), a portion of microbiological contaminants such as endotoxin, viruses and bacteria, and particles. Because of this broad spectrum of solute rejection, reverse osmosis is an important process in a wide variety of water treatment processes. The removal of inorganic contaminants by reverse osmosis membranes has been studied in great detail by many researchers using a variety of membrane types. Complex interactions occur in feed waters containing mixtures of ionic species. Nevertheless, general guidelines for the rejection of inorganic contaminants by reverse osmosis membranes can be given:
Ionic contaminants are more readily rejected than neutral species. For most membrane types, polyvalent ions are rejected to a greater extent than monovalent ions. If the polyvalent ion is strongly hydrated, rejection is even higher. An example of this interaction is that of sodium. Sodium as sulfate (Na2SO4) has a higher rejection than when present as sodium chloride (NaCl), because the divalent sulfate ion is rejected to a greater extent than the monovalent chloride ion.
Because electrical neutrality must be preserved, ions diffuse across the membrane as a cation-anion pair. As a consequence, rejection of a particular ion depends on the rejection of its counter ion. Variations in pH influence the water flux and rejection characteristics of reverse osmosis membranes exposed to a mixture of monovalent and polyvalent solutes. This effect of pH varies with membrane composition and ionic species. For example, fluoride rejection increases from 45% to 90% as pH increases from 5.5 to 7.2, whereas nitrate rejection decreases slightly as pH increases from 5.2 to 7.0. In instances when pH has exceeded 9, and the water contained chloramines, a decreased rejection of solutes by polyamide thin film composite membranes has been observed.
High pH causes chloramines to dissociate into ammonium and hypochlorite ions. The ammonium ions are poorly removed by activated carbon, and interact with the polyamide membranes, causing their rejection characteristics to deteriorate. The decrease in rejection can generally be reversed by lowering the pH of the water supply. Larger municipal water systems are now using chloramines to treat water instead of using free chlorine. This dramatically reduces membrane performance and product life. Inorganic contaminants with higher molecular weights (greater than 200) are rejected to a greater extent than small molecular weight inorganic solutes.
The variable to poor removal characteristics of organic compounds via reverse osmosis membranes dictates the use of auxiliary carbon filtration components either before or after (or both) the membrane. As in steam distillation having similar problems with organic materials, both reverse osmosis and distillation require some type of organic removal mechanism such as replaceable carbon filters. The placement of carbon filters in reverse osmosis systems depends on the type of membrane in use: for cellulose acetate or cellulose triacetate membranes the carbon element is usually placed after the membrane and captive air tank, and immediately before the dispensing faucet. For thin film membranes, a carbon filter is usually placed before and after the membrane. The carbon filter placed in front of the membrane is necessary since various types of organic materials and chlorine are detrimental to the structure of the thin film membrane. Extra caution must be taken regularly to replace the carbon pre filter ensuring reasonable performance and lifetime for the TFC membrane.
Removal of microbiological contaminants: Reverse osmosis manufacturers claim to reduce levels of bacterial and viral contamination in the feed water by factors of 103 to 105. However, in reality reverse osmosis should not be relied upon to produce sterile, much less water with reduced bacterial levels. Using the biological process called mitosis, bacteria and viruses may rapidly penetrate the reverse osmosis membrane through defects and imperfections in the membrane and through tiny leaks in seals of the membrane module. To prevent microbial colonization of the product water side with bacteria and proliferation of these bacteria, regular disinfection procedures may be necessary.
Reverse osmosis is particularly used for drinking water purification. These drinking water purification systems, including a reverse osmosis step, are commonly used for improving water for drinking and cooking. Such systems typically include four or five stages:                1) a sediment filter to trap particles including rust and calcium carbonate        2) optionally a second sediment filter with smaller pores        3) an activated carbon filter to trap organic chemicals, and chlorine ultimately attacking and degrading TFC reverse osmosis membranes        4) a reverse osmosis (RO) filter being a thin film composite membrane (TFM or TFC)        5) optionally a second carbon filter to capture those chemicals not removed by the RO membrane.        6) optionally an ultra-violet lamp is used for disinfection of any microbes escaping filtration by the reverse osmosis membrane.        
In some systems, the carbon pre-filter is omitted and cellulose triacetate membrane (CTA) is used. The CTA membrane is prone to rotting unless protected by the chlorinated water, while the TFC membrane is prone to breaking down under the influence of chlorine. In CTA systems, a carbon post-filter is needed to remove chlorine from the final product water.
Reverse osmosis is used for a variety of water purification processes to treat storm waters, wastewater, desalinate ocean water, and for various industrial applications. In all of these applications, pre-treatment is important when working with RO and nanofiltration (NF) membranes due to the nature of their spiral wound design. The material is engineered to allow only one way flow through the system. Consequently, the spiral wound design doesn't allow for back pulsing with water or air agitation to scour its surface and remove solids. Since accumulated material cannot be removed from the membrane surface systems they are highly susceptible to fouling (loss of production capacity). Therefore, pretreatment is a necessity for any RO or NF system. Pretreatment in Seawater reverse osmosis systems has four major components:
1. Screening of solids. Solids within the water must be removed and the water treated to prevent fouling of the membranes by fine particle or biological growth, and reduce the risk of damage to high-pressure pump components.
2. Screening of biologicals
3. Prefiltration pH adjustment. If the pH of upstream saline water is above 5.8 in the acidic-alkaline measurement scale, sulfuric acid or other acidic solution is used to adjust the pH of water at 5.5 to 5.8.
4. Cartridge filtration. In these reverse osmosis systems, a pump supplies the pressure needed to push water through the membrane, even as the membrane rejects the passage of salt through it. Typical pressures for brackish water range from 225 to 375 lbf/in2 (1.6 to 2.6 MPa). In the case of seawater, they range from 800 to 1,180 lbf/in2 (6 to 8 MPa). The membrane assembly consists of a pressure vessel with a membrane allowing feed water to be pressed against it. The membrane must be strong enough to withstand whatever pressure is applied against it. RO membranes are made in a variety of configurations, with the two most common configurations being spiral-wound and a hollow-fiber. Liming material is used to adjust pH at 6.8 to 8.1 to meet the potable water specifications. Post-treatment consists of stabilizing the water and preparing for distribution. Disinfection (sometimes called germicidal or bactericidal) is employed to kill the bacteria or other organisms in the products by means of ultraviolet radiation, using UV lamps directly on the product.
Prefiltration of high fouling waters with another, larger-pore membrane with less hydraulic energy requirement, has been evaluated and sometimes used since the 1970s. However, this means the water passes through two membranes and is often repressurized, requiring more energy input in the system, increasing the cost. Other recent development work has focused on integrating RO with electro dialysis to improve recovery of valuable deionized products or minimize concentrate volume requiring discharge or disposal.
Typical water treatment using membrane separation employs them in a series of sequential membrane configurations, such as spiral or stacked filtration designs after larger particles have been removed. For example, reverse osmosis membranes are generally employed as the last step after larger particle membrane filtration has occurred. In a conventional water treatment filtration system, particle filtration is first used to remove larger particles. Thereafter successively finer and finer membrane filters are employed to remove finer particles. Membrane pore sizes can vary from 1 to 50,000 angstroms depending on filter type. “Particle filtration” removes particles of 10,000 angstroms or larger. Microfiltration removes particles of 500 angstroms or larger. “Ultrafiltration” removes particles of roughly 30 angstroms or larger. “Nanofiltration” removes particles of 10 angstroms or larger. Reverse osmosis is in the final category of membrane filtration, “Hyperfiltration,” and removes particles larger than 1 angstrom.
Reverse osmosis membranes essentially retain the salts and organic compounds and essentially pass only water and molecules in the range of 5 Angstroms (0.005 microns). Since essentially all dissolved and suspended material is rejected by the membrane, the RO permeate is essentially pure water. This sequence is shown below:

Membranes accomplish a great deal in water purification systems, including: ion removal, particulate removal, removal of organic compounds, and organism removal. Membranes range dramatically in pore size, molecular weight cut off, and ion rejection. Ion removal membranes are at the “tight” end of the spectrum and include reverse osmosis (RO) membranes, and nanofiltration membranes. The membrane chemistry has become refined to allow the rejection percentage to be specified anywhere between 99.9% and 50%, blurring the distinction between nanofiltration, low pressure, standard rejection, and high rejection RO membranes. A major distinction remains between cellulose based and non-cellulosic membranes. Cellulosic membranes tolerate exposure to bactericidal oxidizing agents and in fact must operate with a disinfectant present because organisms will eat the membrane material. Although it may be seen as an advantage to allow a chlorine residual to remain in the water through the reverse osmosis process, the advantages of non-cellulosic membranes far outweigh this advantage. Non-cellulosic membranes operate at much lower pressures and can tolerate a broad range of pH. In addition, all the advanced formulations are in non-cellulosic membranes. One of the most important characteristics of ion removal membranes is they will reject a certain percentage of ions no matter how high in ion concentration the feed stream is (up to maximum osmotic pressure). This is a significant advantage over ion exchange requiring exchange of every ion it removes. This characteristic virtually mandates inclusion of membrane separation in every ion removal system. It is rarely economically feasible to utilize ion exchange alone for ion removal. The primary decision in applying membrane separation is whether to use a single pass system or a double pass system.
For example, according to Koch Membrane Systems, Inc. (KMS), water can be softened with a nanofiltration (NF) membrane rejecting 85% of salt (sodium chloride) but 99% of the hardness ions (calcium and magnesium). The highest salt rejection rates (99.7% or higher), potentially provided by RO membranes, are required for seawater desalinization. KMS reverse osmosis products are designed for cross-flow separation, where a feed stream is introduced into the membrane element under pressure and passed over the membrane surface in a controlled flow path. A portion of the feed passes through the membrane and is called permeate. The rejected materials are flushed away in a stream called the concentrate. Cross-flow membrane filtration uses a high cross flow rate to enhance permeate passage and reduce membrane fouling.
KMS' cross-flow membrane filtration controls the effect of concentration polarization and the gel layer. It provides continuous membrane filtration. Its reverse osmosis process is a moderate to high pressure (80-1200 psig) driven process for separating larger size solutes from aqueous solutions by means of a semi-permeable membrane. This process is carried out by flowing a process solution along a membrane surface under pressure. Retained solutes (such as particulate matter and dissolved salts) leave with the flowing process stream and do not accumulate on the membrane surface. The amount of salt and other impurities is often referred to as TDS, or total dissolved solids. The higher the TDS, the more feed pressure required.
KMS membranes are made with various rejection rates for different applications. KMS produces membrane polymers to cover the full spectrum of recovery osmosis pressure ranges. One of these membrane types is the TFC (thin-film-composite) family. TFC-S (“S” for softening) and TFC-SR (“SR” for selective rejection) are ideal for low TDS or softening type applications. For higher purity permeate, the ULP (ultra-low pressure) line offers high water flux and salt rejection in a 125 psi class membrane.
When salt rejection is paramount, the TFC-HR (high rejection) elements offer 99.5% in standard form, or 99.7% in the Premium model. Typically used in brackish water applications with up to 2000-5000 mg/L TDS, they operate around 200 psi When the water is higher in TDS, the XR (extreme rejection) element comes into its own with 99.7% salt rejection and excellent silica and TOC (total organic carbon) removal too.
For seawater desalination, KMS offers the SS (single-stage) membrane in various sizes and configurations allowing optimum system design. In the Premium line, salt rejection is at least 99.75%. Many of the SS products are designed to run up to 1200 psi for high salinity/high recovery installations.
The cleaning system is an important part of a desalination installation. Membranes can become contaminated after they have been used for some time, with pollutants such as colloids, bio films and biological matter. These contaminants can absorb to the membrane surface and the pipes of the membrane system and consequentially, the performance of the system will decrease. The system may even be seriously damaged. Consequently, a system needs cleaning periodically. Cleaning of a Reverse Osmosis system is usually started when the following conditions are in consecutive order:
1. The normalized flux has decreased 10-15%.
2. The normalized salt content of the permeate has increased 10%.
3. The pressure gradient in a pressure vessel has decreased 15%
The cleaning procedure of a reverse osmosis system consists of the following process steps according to Siemens:
1. Production of the cleansing fluid. The fluids used for the cleaning process need to be of a certain pH and all chemicals must be dissolved and mixed before the cleansing fluid is added in the membrane elements.
2. The removal of feed water from pressure vessels and elements with cleansing fluid.
3. Low-flow recirculation through pressure vessels. The cleansing fluid is now in the drains and the feed water has been forced out of the system.
4. Soaking in the cleansing fluid. The pump is shut off and the cleansing fluid will soak into the membranes.
5. Drainage of the pressure vessels. The applied cleansing fluid is pumped out of the system. By sampling the cleansing fluid and analyzing the samples, one can determine the amount of contamination.
6. Rinsing out the system. For the rinsing process, either permeates or good-quality water is used.
7. Starting up the cleaned system parts. The installation is started up according to the usual process parameters. When cleansing fluid is present in the permeate, the system needs to be rinsed repeatedly, until permeate quality is satisfactory.
Reverse Osmosis membranes and other membrane systems thus need periodic cleaning and servicing. For optimal performance specific chemicals are required, depending on the cause of the pollution. The following are general cleaning problems encountered:
Scaling
Scaling is concerned with the seclusion of suspended inorganic particles, such as calcium carbonate, barium sulfate and iron compounds. This mainly occurs in hard water.
Fouling
Fouling is concerned with the seclusion of organic, colloidal and suspended particles.
Bacteria and other microorganisms decomposing these particles will create substrates. As a consequence they will grow and develop forming biofilms on the membrane. In addition, oil and organic accumulation can occur.
Iron Deposits
With waters containing high concentrations of iron, it is periodically necessary to remove iron containing deposits.
These water scaling, fouling, and iron deposit processes will cause a decrease in capacity and/or an increase of the pressure and, as a result, of the energy use. This is due primarily because the filtration process causes these membranes to become plugged with mineral scaling and/or from microbial fouling. And as a result, both have a direct and inverse affect on performance and operational costs—it reduces the volume of water potentially filtered and increases the amount of energy required to process it. In addition to these costs, are the costs related with disposal of the materials collected (brine).
It is thus important to purify the membrane preventively. In many cases regular mild cleaning is better than cleaning periodically with an aggressive cleaning product to insure the membrane will last longer. Reverse osmosis membrane cleaning occurs on and off site. However, both methods are expensive and are to be minimized if possible.
Suda et al. (JP 63039686 A, Feb. 20, 1988—the Derwent abstract, the esp@cenet abstract, uses SO2 for disinfection and reduction of scaling and iron deposits on reverse osmosis filters, but it does not address soil applications and total recovery of the water entering a reverse osmosis system via brine monitoring and conditioning of the retentates to insure the brine levels do not exceed levels for plant irrigation application via SO2 acidity and lime SAR soil adjustments as well as conditioning the solids and brines for heavy metals removal. Suda et al also fails to disclose the need for increased acidity to open up the soil pores.
El-Shoubary et al. (U.S. Pat. No. 6,096,222) issued Aug. 1, 2000 uses strong phosphoric and sulfuric acids and calcium hydroxide or calcium oxide to adjust the pH of heavy metals contaminated groundwater during the treatment process without increasing the total dissolved solids (TDS). The El-Shoubary et al. method therefore is not designed for use with a reverse osmosis filtration systems as the El-Shoubary et al. strong phosphoric and sulfuric acids are not compatible with many reverse osmosis filters. El-Shoubary et al. is focused on recovering minerals as metallic hydroxide precipitant sludges in a pH range above 7 and preferably 8.3 (Col 3, Lines 24-25). Depending upon the heavy metal to be removed as metal hydroxides, the pH must be raised from 7 to 11. This resulting alkaline solution is not suitable for treating western alkaline soils where the soil pores are plugged with bicarbonates/carbonates from long irrigation use.
Grott (U.S. Pat. No. 6,651,383) issued Nov. 25, 2003 is a method used with wastewater contaminated with greater than 0.15% by weight of the salts of Na, Ca, Mg, Cl, SO4, or CO3 or combinations thereof (claim 1, Cols 23, 25). Grott addresses the lime treatment of “wastewater” defined as “any water containing sufficient salts as to have no acceptable use due to costs or contamination levels” (Col 7, lines 9-13. It therefore is used for treatment of overly concentrated brines unsuitable for irrigating crops, not pre-treatment of intake wastewaters prior to RO treatment.
Kimura et al., U.S. Pat. No. 6,468,430 issued Oct. 22, 2002 discloses a method for inhibiting growth of bacterial or sterilizing a separation membrane using sulfuric acid at a pH of 4 or lower. This Kimura et al. method uses various mineral acids, which increase the ions added to the system, possibly increasing the osmotic pressure of the RO separation system and affecting the brine composition for disposal. Baldridge et al., U.S. Pat. No. 7,165,561 issued Jan. 23, 2007 discloses a biofilm reduction method in cross flow filtration membrane systems using enzyme/surfactant compounds. Kimura et al, U.S. Pat. No. 6,743,363 issued Jun. 1, 2004 discloses a method of bacteristasis or disinfection for preselective membranes using sulfuric acid or similar pre-treatment crude water to make the water have a pH of 4 or lower. The Kimura et al '363 method adds sodium bisulfite for preventing permseletive membranes from being degraded by chlorine oxides used for disinfection. This aggravates the saline concentration of the brines by adding more sodium into the reverse osmosis system, which adversely affects their use for land application, and the increased ions may also increase the osmotic pressure of the RO separation system. Schacht et al, U.S. Pat. No. 7,220,358 issued May 22, 2007 discloses a method for treating membranes and separation facilities utilizing multiphase gas/liquid flow cleaning and treatment. The method employs a pre-rinse alkaline wash using caustic, enzymes, surfactants, chlorine and sequestraints; an acid was of nitric, phosphoric, citric acids and surfactants, an alkaline wash using caustic, enzymes, surfactants, chlorine and sequestraints, and a water rinse. Optionally, an antimicrobial treatment with chlorine, peracetic acid, and hydrogen peroxide can be used. In addition a sodium bisulfite, citric acid, and lactic acid preservation may be added to minimize microbial growth. Again, the addition of these added ions may affect the osmotic pressure of the RO separation system, and adversely affect the brine composition of the retentates for disposal purposes.
Cited for general interest is Zeiher et al., U.S. Pat. No. 7,060,136 issued Jun. 13, 2006 disclosing a method of monitoring membrane cleaning process using measurable amounts of inert fluorescent tracers added to a membrane cleaning process stream to evaluate and/or control the removal of contaminants and/or impurities during cleaning.
Acidic treated streams are often amended with gypsum to add calcium; see Cecala et al., U.S. Pat. No. 6,979,116 issued Dec. 27, 2005; and Cecala et al., U.S. Pat. No. 7,147,361 issued Dec. 12, 2006. Gypsum, or calcium sulfate (CaSO4) is a pH neutral material, which will not raise the overall pH of water. Gypsum is primarily used to increase the overall amount of calcium to counter excessive amounts of sodium that may be present in water. Murphy, U.S. Pat. No. 4,806,264 issued Feb. 21, 1989 discloses a method of selectively removing selenium ions from an aqueous solution using ferrous ions in the form of ferrous hydroxide.
While reverse osmosis water treatment systems do work, the biggest hurdle and drawback is the high cost associated with operating and maintaining such a system and disposing of the brines. Currently, well operated reverse osmosis systems separating sea water produce 1 gallon of concentrated brines for every gallon of pure water filtrate retained. The method described below provides a sulfurous acid pre-treatment method, which reduces membrane fouling and produce a metal free filtrate, which can be disposed of without concern for metals contamination.